Useless? No. Larger wire typically also has more weight, so it may make it "fish" differently.

The big thing is perspective- if the wire is WAY LARGER than the other aspects of the fly, say approaching the diameter of the hook wire, then obviously, it won't look correct when moving through clear as glass, slow water to a fish who has the time to inspect it... but if its in rapidly moving, cloudy or roiling water, and the fish is hungry, and it REASONABLY represents what they normally see in the food chain... its unlikely it will be refused.